Let's make this clear - IT IS WHOLLY UP TO AMAZON'S WHIM WHEN YOU'RE ALLOWED TO CONTACT THESE FOLLOWERS. You aren't allowed to email them every day harassing them :). Amazon retains an extremely tight control over the contact feature. It's not even available with each new release. Even though I can sometimes release several books in a single week, sometimes it's two months or more between Amazon's offerings of having me contact those followers.
Here's how it works.
Sometime after you load a new book into their system - maybe days, maybe weeks - Amazon will send you a strange email message. Strange because the email subject isn't something useful like "Tell your Followers about your Release of Book XYZZY." No, instead their subject line reads, "A Question about your Book: XYZZY" which makes it sound like something might be wrong.
The message shows you the title and the cover of the book in question. If you click "yes" you get taken to the Amazon.com website with a tiny box in which to type your message to those followers.
I highly recommend writing your message IN WORD (or something else with a spell checker) and making sure it says exactly what you want it to say. Re-read it multiple times. Word's spell checker misses all sorts of issues! You want to make sure that your message is as professional as possible. This message, just like your blurbs, is your demonstration of how well you write and why a reader would want to read an entire book written by you. Make sure it shines.
When you post this - it goes out! No second chances. So make sure it's perfect.
On this confirmation screen you can click the "read more" link to expand the page, in order to see the whole message you sent.
And you're done! Now you keep working on your next book and then hopefully you'll get another follower-contact option when you release that one. But don't count 100% on having that option. Amazon works in strange and mysterious ways and it is wholly up to them when they give you that contact opportunity.
If you don't yet have an Author Central page set up, you absolutely should. Here are the details.
Setting Up an Amazon Author Central Page
Lisa Shea's Amazon Author Central Page
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